The present invention relates to a lockup clutch of a torque converter for an automatic transmission of a motor vehicle, and more particularly to a system for controlling the lockup clutch.
A lockup clutch of a torque converter is disposed in a converter cover. The lockup clutch has a lockup piston and is brought into lockup state or release state by controlling oil pressures applied to a release chamber and to an apply chamber on opposite sides of the piston so as to engage and disengage the piston with respect to the converter cover. When the lockup clutch is engaged, the power of an engine is directly transmitted to the automatic transmission.
However, if the engine speed becomes low in the lockup state, the variation of the engine speed in accordance with a change of the engine torque is transmitted to the transmission to cause oscillation and noise.
At a low speed range of the vehicle, since the output power of the engine and the transmitting torque are small, the torque capacity of the lockup clutch can be reduced to avoid the occurrence of oscillation and noise. In order to reduce the torque of the lockup clutch, the lockup clutch is controlled to temporarily slip in accordance with variation of the engine speed and the engine torque. Thus, the transmission of the engine speed is reduced to effectively reduce the oscillation and noise.
Japanese Patent Application Laid-Open 57-163732 discloses a system for controlling a lockup clutch by causing the clutch to slip. The system has a torque variation detecting device for detecting the frequency of the torque variation and for controlling the slip of the lockup clutch in accordance with the torque variation.
However, in such a system, since a torque variation detecting device is employed, the system becomes expensive. Further, since the slipping is controlled in response to the frequency of the torque variation, it is difficult to properly control the slipping of the clutch in accordance with the magnitude of the variation.